In extracorporeal blood treatment, e.g. hemodialysis, hemofiltration, hemodiafiltration etc., an apparatus for preparing dialysis fluid is used which prepares the required dialysis fluid from the basic components of water, basic fluid and acid fluid. During the blood treatment of a patient the prepared dialysis fluid is passed through the dialysis-side chamber of a dialyzer through the semi-permeable membrane of which toxic substances and water are absorbed from the blood passed through the blood-side chamber of the dialyzer by diffusion (hemodialysis) or diffusion in combination with convection (hemofiltration and hemodiafiltration).
During the extracorporeal blood treatment, the dialysis fluid flushes the patient's blood to be treated in the dialyzer. The basic component of the dialysis fluid usually is a substrate containing sodium hydrogen carbonate (NaHCO3) and the second component (SK) usually is a solution containing sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl2), calcium chloride (CaCl2)), glucose (C6H12O6) and acetic acid (CH3COOH) and/or citric acid (C6H8O7).
For preparing and, respectively, for proportioning the dialysis fluid, usually metering pumps and conductivity probes are employed. A probe measures the conductivity of the NaHCO3 after addition thereof with a first metering pump (BICLF). After addition of the acid component with a further metering pump, another probe measures the conductivity of the entire dialysis fluid (ENDLF).
In the case of conductivity-controlled proportioning the amounts to be added are regulated by way of the conductivities measured. In the case of volumetric proportioning, the conductivity probes merely serve for checking as the proportioning is performed directly via the metering pump delivery rates. However, this requires the exact knowledge of the composition of the components used.